1. Technical Field
The present disclosure relates to a millimeter wave antenna and a radar apparatus for vehicle.
2. Description of the Related Art
Millimeter wave antennas are used for radar apparatuses for vehicle, for example. Such a radar apparatus for vehicle is mounted in a vehicle, which is a mobile unit, and detects, for example, a direction, relative distance, and relative velocity of an obstacle, such as a mobile unit running ahead.
In recent years, prevention safety technologies for preventing accidents have been particularly attracting more and more attention. ACC (Adaptive Cruise Control system) that uses a 76 GHz band millimeter wave radar and ADAS (Advanced Driver Assistance System) such as a pre-crash safety system are already becoming standard equipment on luxury cars. In response to such increasing needs, standardization of a new radar band (79 GHz band) is being promoted, and the new radar band is expected to become available from fiscal year 2015.
The millimeter wave radar sends a millimeter electromagnetic wave in a desired direction, receives its reflected wave, and previously detects an object that can be an obstacle. In the millimeter wave radar, a planar patch antenna or a slot antenna formed in a waveguide transmits and receives an electromagnetic wave. A location best suited to install the antenna is considered an inside of a bumper that always faces a traveling direction of a vehicle, among nonmetallic exterior bodies that can pass the electromagnetic wave. Since the inside of a vehicle bumper is typically not a sealed space, rain or dust enters. Accordingly, in order to protect an antenna body, it is necessary to provide a radome for covering a front of a fixing portion to which the antenna body is attached. The radome itself also needs to have a certain amount of thickness and strength to secure durability and sealing performance.
In the radar apparatus for vehicle configured as describe above, a radar wave radiated from the antenna body passes through the radome and the bumper, and is radiated toward a survey object, and a reflected wave from the survey object will pass through the bumper and the radome, and will reach the antenna body.
Accordingly, the electromagnetic wave radiated from the antenna body passes through each of the radome and the bumper twice by returning to the antenna body, causing a problem of reflection loss and absorption loss resulting from the radome and the bumper. In a short-wavelength millimeter wave band in particular, influence of the reflection loss and the absorption loss is great. In order to solve such a problem, Patent Literature 1 (Japanese Patent No. 4,065,268) and Patent Literature 2 (Unexamined Japanese Patent Publication No. 2003-10676) propose specifying a positional relationship between the radome and the bumper based on an electric length.